What is 256-bit?


256-bit (2256) is a measure of computer information and an attribute of computer systems. It makes awfully large numbers so it is a common length for cryptography keys that we don't want people to be able to guess or calculate.


How big is 256-bits?

256-bits is too big for us to even imagine!

We're fooled by the integers we can see and understand: "2" and "256" seem so reasonable but every 210 is equivalent to 103 or 10001. That means 2256 is about 10168 (1 with 168 zeroes following) or 100026. So let's have a look at the world in the universe that we can see around us:

  • All the data in the world was estimated at 4.4 Zettabytes (273 bits) in 2013. That is more than a 64-bit number could address. If each of those bit were duplicated 273 times we would finally have a number larger than 128-bits. We'd get to 146 bits, yet we'd still be a long way from the range addressable with 256-bits.
  • What about stars in the universe? One estimate is an unimaginable 7 x 1022 stars. A Zettabyte is 1021 so this estimate is only one magnitude of difference from all the data in the world. Yet the gulf between this and 256-bits remains immense.
  • If we consider the number of atoms in the universe then we creep a little close at 1082. If 96% of the universe is dark matter then that would help a lot … not. We would only get up to about 1085.



Common uses of 256-bits
Category Description

No general purpose CPUs are 256-bit but there are general purpose CPUs which have special extensions that use 256-bit instructions. Often the instructions consist of several narrower instructions to be performed in parallel.

CPU support

It is common for video graphics cards to have 256-bit memory bus widths. Here's an example. Technically, that is the aggregate of what are usually 32-bit interfaces to each memory module as in the diagrams in this article.

Operating software


Communications and networking









Cryptography is the main use of bit-lengths 128-bit, 256-bit, and higher. 128-bit is generally the minimum recommended for a private key using a secure algorithm like AES or Triple-DES. Therefore 256-bit would be recommended to ensure longer-term security. Remember that if you are using public keys then the key length will need to be much longer.


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